Restricted service telephone system



Patented Aug. 22, 1950 RESTRICTED SERVICE TELEZHONE sysTEM John Wicks, Bilcxi, Miss., assignor to Automatic Electric Laboratories, Inc., Ohicago, Ill, a cor.-

Poration of Delaware Application May 22, 1947, Serial No. 7 19,7513

7 LClaims. l

The present invention relates to automatic telephone systems and more particularly to such systems in which different groups of subscriber substations are rendered correspondin different classes of service.

In an automatic telephone system serving a central ofiice area and the adjacent suburban areas, calls from the subscriber substations in the central ofliee area that are to be completed to subscriber substations in the suburban areas are usually first extended to an operator switchboard in the central office area and then routed by the operators thcreat to the subscriber substations in the suburban areas. While this system is entirely satisfactory in operation, a large switchboard in the central ofilce, staffed by a great number of operators, is required in order to complete the calls from the central ofi'ifie .area to the surburban areas.

Accordingly, it is an object of the present in.- vention to provide an automatic telephone system comprising an improved switching arrangement; whereby subscriber substations of a first class in the central ofiice are rendered a corresponding cla of service permitting them to set up connections to subscriber substations in the suburban areas, and subscriber substations of a second class in the central ofiice are rendered a corresponding class of service preventing them from setting .up connections to subscriber substations in the surburban areas.

Another object of the invention is to provide an automatic telephone system of the character described; wherein the first and second classes of service mentioned are rendered to the corresponding classes ,of subscriber substations regardless of whether the subscriber substations are connected to individual or party lines.

Another object of the invention is to provide an automatic telephone system of the character described that includes subscriber substations of first and second classes that are connected to the same party line and rendered correspondin first and second classes of service.

A further object .of the invention is to provide an automatic telephone system in which different classes of service are rendered to subscriber substations; wherein the classes of service are selectively established by an improved arrangement including apparatus provided atthe subscriber substations.

A further object of the invention is to provide an automatic telephone system in which different classes of service are rendered to subscriber sub stations'; wherein the classes .of service are selec- 2 tively established by an improved arrangement including difierent types of digit transmitting mechanism provided at the subscriber substations.

A still further object of the invention is to provide in a telephone system including a plurality of automatic switching stages, wherein an advanced switching stage supplies current of a polarity opposite to that of a preceding switching stage, an improved arrangement comprising an asymmetrical current conducting device disposed at subscriber substations rendered restricted service for preventing the subscriber substations mentioned from controlling operation of the advanoed switching stage.

Further features of the invention pertain to the parti ular arrangement of the circuit elements of the system, whereby the above-outlined and additional operating features thereof are attained.

The invention both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specifications taken in connection with the accompanying drawing in which the single figure illustrates the details of an automatic telephone system having incorporated therein the features of the present invention.

Referring now more particularly to the drawing, there is illustrated automatic telephone system comprising a central oflice lb and ,a remote exchange 25. The central office ill is connected to the remote exchange 293 by a group of trunk lines including the trunk line M0.

The central office it may serve a group of several hundred subscriber lines, including both private or individual subscriber lines and party subscriber lines, provided with subscriber substations rendered two classes of service, including non-restricted service and restricted service. A first group of these subscriber lines includes the individual subscriber lines 39 and 48 and the party subscriber line H9. The individual subscriber lines 99 and 45 have the subscriber substations Tl and-T2 respectively connected thereto and respectively rendered non-restricted and restricted service. The party subscriber line H 9 has the subscriber substation PT! and PI? commonly connected thereto and respectively rendered non-restricted and restricted service. The subscriber substations Tl, PT-l, etc., rendered non-restricted service, are provided with identical and conventional substation apparatus; while the subscriber substations T2, PTZ, etc, rendered restricted service, are provided with identical and unique substation apparatus. For example, the subscriber substation TI rendered non-restricted service is provided with substation apparatus including a circuit, a telephone instrument II, a hookswitch i2 and a dial I3; the dial I3 including a pair of impulsing springs I4 serially arranged in the substation circuit connected to the line conductors C3I and C32 of the subscriber line Similarly the subscriber substation T2 rendered restricted. service is provided with substation apparatus including a circuit, a telephone instrument 2!, a hookswitch 22 and a dial 23; the dial 23 including a pair of impulsing springs 24 serially arranged in the substation circuit connected to the line conductors CM and C42 of the subscriber line 45. Also the substation apparatus at the subscriber substation T2 includes an asymmetrical current conducting device 25 in the form of a copper oxide rectifier bridging the impulsing springs 24 of the associated dial 23 and arranged to block the passage therethrough of substantial current of a predetermined polarity. Specifically, the device 25 blocks the passage therethrough of substantial current when the line conductors C4! and C42 of the circuit line are respectively negative and positive; and permits the passage therethrough of substantial current when the line conductors CM and C42 of the subscriber line 40 are respectively positive and negative. When the line conductors of the subscriber line 40 are so poled that the passage of substantial current through the device 25 is blocked the impulsing springs 24 of the dial 23 are capable of transmitting the impulses of a digit over the subscriber line 40 incident to operation of the dial 23. On the other hand, when the line conductors of the subscriber line 40 are so poled that the passage of substantial current through the device 25 is permitted the impulsing springs 24 of the dial 23 are effectively shortcircuited and are incapable of transmitting the impulses of a digit over the subscriber line 40 incident to the operation of the dial 23. Also,

each subscriber line is terminated in a line switch rality of groups of first selectors, the first group of first selectors including the first selector terminating the trunk accessible to the linev switches 35, 45, I15, etc., associated with the subscriber lines in the first group previously mentioned. Also the central ofiice I0 comprises a plurality of groups of local connectors, the first group of local connectors including the local connector I0 terminating the trunk 80 accessible to the various groups of first selectors. The localconnectors it, etc., in the first group have access to the subscriber lines 35, 48, I10, etc., in the first group. Further, the central ofiice I0 comprises a group of second selectors including the second selector I I0 terminating the trunk I35 accessible to the different groups of first selectors.

Further, the central office It! comprises a group of trunk circuits terminating the trunk lines in the group extending to the remote exchange 25; the trunk circuit l5il terminating the trunk line E45 and provided with a trunk i60 accessible to the second selectors I Ill, etc. Also the central ofiice it comprises an operator position I00 terminating a group of trunks including the trunk 55. The trunks 96, etc., are terminated by trunk circuits 535, etc., respectively provided with incom ing trunks 90, etc., accessible to the difierent groups of first selectors.

The remote exchange 20 serves a group of subscriber substations including the subscriber substation T3; and comprises automatic switching apparatus I45 of the Strowger type terminating the trunk lines I40, etc., and having access to the subscriber lines I46, etc., respectively extending to the subscriber substations T3, etc. Also the switching apparatus I45 is accessible to the subscriber substation T3, etc., via the subscriber lines, I46, etc.

Preferably in the central ofiice I0 each of the first selectors is identical to the first selector 53 which is of the conventional Strowger type including a line relay R5l and a switch through relay R55. Preferably, each of the local connectors is identical to the local connector I3 which is of the conventional Strowger type including a line relay RH and a reverse battery relay R15. Preferably, each of the second selectors is identical to the second selector IIEJ which is of the conventional Strowger type including a line relay RI I I, a switch through relay R and a signal relay RIZU, as well as a timer I25.

A better understanding of the connection and arrangement of the telephone system will be facilitated from the following description of the operation of the apparatus incorporated therein incident to the extension of various connections from the subscriber substations in the central ofllce I'U.

Local calls in the central ofiice 10 First assuming that a call is initiated at any subscriber substation in the central office I6, such, for example, as the subscriber substation TI rendered non-restricted service; when the call is initiated at the subscriber substation TI by the removal of the telephone instrument II thereat from its associated switchhook, the hookswitch I2 is closed completing an obvious connection including the impulsing springs I4 of the dial I3 across the line conductors C3I and C32 of the subscriber line 30 extending to the line switch 35. Operation of the line switch 35 is initiated, whereby it operates automatically to select a trunk extending to an idle first selector in the associated group. For example, the line switch 35 may select the trunk 60' extending to the first selector 5!]; thereby to complete a loop circuit between the calling subscriber substation Ti and the line relay RSI in the first selector 50. More particularly, ground potential is applied by way of the lower winding of the line relay R51 and the contacts 59 to the line conductor C62 of the trunk 60 and by Way of the line switch 35 to the line conductor C32 of the subscriber line 39; and battery potential is applied by way of the upper Winding of the line relay RSI and the contacts 56 to the line conductor C6| 0f the trunk 60 and by way of the line switch 35 to the line conductor C3I' of the subscriber line 30. When this loop circuit is completed, the line relay R5I in the first selector 5!] is operated in order to condition the first selector 50 to be responsive to the first digit dialed at the calling subscriber substation TI.

The subscriber at the calling subscriber substation TI then proceeds to operate the dial I3 in accordance with the first digit of the directory number of the called subscriber substation; whereby the impulsing springs I4 of the dial [3 are controlled to transmit a corresponding number of impulses over the above traced loop circuit to the line relay R! in the first selector 50. The line relay B55 in the first selector 5i! follows the first digit bringing about operation of the first selector 58 to select the corresponding group of local connectors and then an idle local connector therein. For example, the first selector 55 may be operated to select the trunk 8i} extending to the local connector 36; whereby the switch through relay R55 in th first selector to is operated to interrupt, at the contacts 56 and 59, the previously traced loop circuit extending between the calling subscriber substation Ti and the line relay BM in the first selector 5b; and to com plete, at the contacts bl and 58, an alternative loop circuit extending between the calling subscriber substation T! and the line relay R7: in the local connector 79. The loop circuit mentioned extends from ground potential by way of the lower winding of the line relay RH and the contacts '59 to one of the line conductors of the trunk 5353 and thence by way of the contacts 58 to the line conductor 082 of the trunk 8t and thence by way of the line switch 35 to the line conductor C32 of the subscriber line 88 and then to the calling subscriber substation Ti; and from battery potential by way of the upper winding of the line relay RIM and the contacts H to the other line conductor of the trunk 89 and thence by way of the contacts 5'! to the line conductor C3! of the trunk 38 and thence by way of the "he switch 35 to the line conductor C3! of the subscriber line Bil and then to the calling subscriber substation Tl. When this alternative loop circuit is completed, the line relay Rli in the local connector it operates in order to condition the local connector ill to be responsive to the next two digits of the directory number of the called subscriber substation dialed at the calling subscriber substation Ti. Assuming that the present call is to be extended to the called subscriber substation Til, the subscriber at the calling subscriber substation Ti then proceeds to operate the dial is in accordance with the next two digits of the directory number of the called subscriber substation T2; whereby the line relay Rli in the 4 local connector it follows the two digits men tioned. Specifically, the local connector iii openates first to select the group of subscriber line. including the subscriber line 38 extending to the called subscriber substation T2 and then to select the particular subscriber line as in the previously selected group. The subsequent operation of the local connector l5 depends upon the idle or busy condition of the called subscriber substation T2 at this time in accordance with conventional practice.

Assuming that the called subscriber substation T2 is idle at this time, ringing current is projected from the local connector Ill over the subscriber line ill; and when the subscriber at the called subscriber substation T2 answers the call by removal or" the telephone instrument 2i thereat from its associated switchhook, the hookswitch 22 is closed completing an obvious connection including the impulsing springs 24 of the dial 23 across 0%2 of the subscriber line 49. When this connection is thus completed, the local connector "it operates to interrupt the projection of ringing current over the subscriber line 49 and switches through in order to bring about operation of the reverse battery relay R therein. Upon operating the reverse battery relay R15 reverses, at the contacts 76, ll, 13, and 19, the polarity of the trunk St with respect to the upper (9,, switch 45. the line conductors CAI and and lower windings of the line relay RH, and, consequently, the polarity of the line conductors C3! and C32 of the subscriber line extending to. the calling subscriber substation TI. This reversal of polarity over the loop circuit extend.- ing between the calling subscriber substation TI and the line relay R1! in the local connector 10 is without particular effect, although it may be employed for metering purposes. At this time an established connection is completed between the calling subscriber substation TI and the called subscriber substation T2.

Subsequently, when the subscriber at the called subscriber substation T2 replaces the receiver of the telephone instrument 2| thereat upon its associated switchhook, the hookswitch 22 is opened interrupting the loop circuit extending between the called subscriber substation T2 and the local connector 70; whereby the reverse battery relay B15 in the local connector 10 restores again effecting the reversal, at the contacts 76, ll, it, and T9, of polarity over the loop circuit extending between the calling subscriber substation TI and the line relay Rll; all without efiect at this time. When the subscriber at the calling subscriber substation Tl replaces the receiver of the telephone instrument thereat upon its associated switchook, the hookswitch I2 is opened interrupting the previously traced loop circuit extending to the line relay R1! in the local connector iii; whereby the line relay Rll restores efiecting the consequent release or the local connector it, the first selector 50 and the line switch At this time the connection between the calling subscriber substation TI and the called subscriber substation T2 is released and the apparatus involved thereat is completely released and available for further use.

In view of the foregoing description of the extension of a local call from the private subscriber substation TI rendered non-restricted service, it will be understood that the extension of a local call from a party subscriber substation rendered non-restricted service will :proceed in an identical manner. By Way of illustration, it is noted that the extension of a local call from the calling party subscriber substation PTI rendered non-restricted service via the party subscriber line I10, the associated line switch I75, the trunk til, the first selector 50, the trunk 80, the local connector 70 and the subscriber line it to the called subscriber substation T2 is identical to that described above.

Now assuming that a call is initiated at a subscriber substation in the central office Ill such. for example, as the subscriber substation T2 rendered restricted service; when the call is initiated at the subscriber substation T2 by the removal of the telephone instrument 2! thereat from its associated switchhook, the hookswitch 22 is closed completing an obvious connection including the impulsing springs 24 of the dial 23 across the line conductors C4! and C 52 of the subscriber line it extending to the line Operation of the line switch 45 is initiated whereby it operates automatically to select a trunk extending to an idle first selector in the associated group. For example, the line switch 65 may select the trunk extending to the first selector 5!]; thereby to complete a loop circuit between the calling subscriber substation T2 and the line relay R5! in the first selector 563. More particularly, ground potential is applied by way of the lower winding of the line relay R5l and the contacts 59 to the line conductor C62 of the trunk 60 and by way of the line switch 45 to the line conductor C42 of the subscriber line 40; and battery potential is applied by way of the upper winding of the line relay Rl and the contacts 56 to the line conductor C61 of the trunk 60 and by way of the line switch 45 to the line conductor C4! of the subscriber line 40. When this loop circuit is completed, the line relay R5! in the first selector 50 is operated in order to condition the first selector 50 to be responsive to the first digit dialed at the calling subscriber substation T2. In passing, it is noted that at the calling subscriber substation T2, the asymetrical current conducting device bridges the impulsing springs 24 of the associated dial 23; however without effect since the polarity of the line conductors C42 and CH of the subscriber line are poled respectively and by the line relay R5| in the first selector whereby the passage of substantial current through the device 25 is blocked so that the impulsing springs 24 of the dial 23 are capable of transmitting the impulses of the first digit dialed at the calling subscriber substation T2 over the subscriber line 40 to the line relay RSI in the first selector 50.

The subscriber at the calling subscriber substation T2 then proceeds to operate the dial 23 in accordance with the first digit of the directory number of the called subscriber substation; whereby the impulsing springs 24 of the dial 23 are controlled to transmit a corresponding number of impulses over the above traced loop circuit to the line relay R5! in the first selector 5B. The line relay R5! in the first selector 5i! follows the first digit bringing about operation of the first selector 50 to select the corresponding group of local connectors and then an idle local connector therein. For example, the first selector 5!] may be operated to select the trunk 80 extending to the local connector 10; whereby the switch through relay R in the first selector 58 is operated to interrupt, at the contacts 55 and 59, the previously traced loop circuit extending between the calling subscriber substation T2 and the line relay R5! in the first selector 50; and to complete, at the contacts 51 and 58, an alternative loop circuit extending between the calling subscriber substation T2 and the line relay R1! in the local connector 10. The loop circuit mentioned extends from ground potential by way of the lower winding of the line relay R1! and the contacts 19 to one of the line conductors of the trunk 80 and thence by way of the contacts 58 to the line conductors CS2 of the trunk 60- and thence by way of the line switch 45 to the line conductor C42 of the subscriber line 40 and then to the calling subscriber substation T2; and from battery potential by way of the upper winding of the line relay RH and the contacts 11 to the other line conductor of the trunk 89 and thence by way of the contacts 51 to the line conductor C6! of the trunk 60 and thence by way of the line switch 45 to the line conductor C4! of the subscriber line 49 and then to the calling subscriber substation T2. When this alternative loop circuit is completed, the line relay R1! in the local connector operates in order to condition the local connector 10 to be responsive to the next two digits of the directory number of the called subscriber substation dialed at the calling subscriber substation T2. Since the line conductors C42 and C4! of the subscriber line 45 are poled respectively and by the line relay R1! in the local connector 10, the passage of substantial current through the device 25 at the calling subscriber substation T2 is blocked so that the impulsing springs 24 of the dial 23 are capable of transmitting the impulses of the next two digits dialed at the calling subscriber substation T2 over the subscriber line 40 to the line relay R1! in the local connector 10.

Assuming that the present call is to be extended to the called subscriber substation TI, the subscriber at the calling subscriber substation T2 then proceeds to operate the dial 23 in accordance with the next two digits of the directory number of the called subscriber substation Tl; whereby the line relay R1] in the local connector 1D follows the two digits mentioned. Specifically, the local connector 10 operates first to select the group of subscriber lines including the subscriber line 30 extending to the called subscriber substation TI and then to select the particular subscriber line 30 in the previously selected group. The subsequent operation of the local connector 1e depends upon the idle or busy condition of the called subscriber substation Tl at this time in accordance with conventional practice.

Assuming that the called subscriber substation TI is idle at this time, ringing current is projected from the local connector 10 over the subscriber line 30; and when the subscriber at the called subscriber substation Tl answers the call by the removal of the telephone instrument ll thereat from its associated switch hook, the hook switch I2 is closed completing an obvious connection including the impulsing springs 14 of the dial 13 across the line conductors C3I and C32 of the subscriber line 30. When this connection is thus completed, the local connector H! operates to interrupt the projection of ringing current over the subscriber line 30 and switches through in Order to bring about operation of the reverse battery relay R15 therein. Upon operating the reverse battery relay R15 reverses, at the contacts 76, 11, i3 and 19, the polarity of the trunk with respect to the upper and lower windings of the line relay RH and, consequently, the polarity of the line conductors C4! and C42 of the subscriber line 40 extending to the calling subscriber substation T2. This reversal of polarity over the loop circuit extending between the calling subscriber substation T2 and the line relay R'H in the local connector 10 poles the line conductors CM and C42 of the subscriber line 40 respectively and whereby substantial current passes through the device 25 at the calling subscriber substation T2 effectively short-circuiting the impulsing springs 24 of the dial 23 in order to prevent the dialing of further digits at the calling subscriber substation T2. However, the short-circuiting of the impulsing springs 24 of the dial 23 at the calling subscriber substation T2 is without particular effect in the present example since no further digits are to be dialed thereat as the call has already been extended to the called subscriber substation Tl. Also this reversal of polarity over the loop circuit extending between the calling subscriber substation T2 and the line relay RH in the local connector 10 may be employed for metering purposes. At this time an established connection is completed between the calling subscriber substation T2 and the called subscriber substation Tl.

Subsequently, when the subscriber at the called subscriber substation TI replaces the receiver of the telephone instrument II thereat upon its associated switchhook, the hookswltch I2 is opened, interrupting the loop circuit extending between the called subscriber substation Ti and the local connector whereby the reverse battery relay B in the local connector m restores again effecting the reversal, at the contacts '86, TI, 18 and 19, of polarity over the loop circuit extending between the calling subscriber sub station T2 and the line relay Rll all Without effect at this time. When the subscriber at the calling subscriber substation T2 replaces the re ceiver of the telephone instrument thereat upon its associated switchhook, the hookswitch 22 is opened interrupting the previously traced loop circuit extending to the line relay R?! in the local connector 76; whereby the line relay Rli restores effecting the consequent releas of the local connector 10, the first selector 50 and the line switch 45. At this time the connection between the calling subscriber substation T2 and the called subscriber substation Ti is released and the apparatus involved therein'is completely released and available for further use.

In view of the foregoing description of the extension of a local call from the private subscriber substation T2 rendered restricted service. it will be understood that the extension of a local call from a party subscriber substation rendered restricted service will proceed in an identical manner. By way of illustration, it is noted that the extension of a local call from the calling party subscriber substation PTZ rendered restricted service via the party subscriber line I79, the associated line switch N5, the trunk 60, the first selector 5!), the trunk 88, the local connector 10 and the subscriber line 30 to the called subscriber substation TI is identical to that described above.

Trunlc calls from the central oflice I0 to the remote exchange First, assume that a call has been initiated at the subscriber substation T! rendered non-restricted service in the central ofiice lil that the line switch 35 has operated to select the trunk 68 extending to the first selector 58; that the first selector 5%] is conditioned to be responsive to the first digit dialed at the calling subscriber substation Tl and that the call is to be extended to a called subscriber substation T3 in the remote exchange Zll. The subscriber at the calling subscriber substation Tl proceeds to dial the first digit of the directory number of the called subscriber substation T3 in the remote exchange 2%; thereby to cause operation of the first selector in order to select a trunk extending to an idle second selector. For example, the first selector 5!) may operate to select the trunk I35 extending to the second selector l H whereupon the switch through relay B55 in the first selector 56 operates to interrupt, at the contacts 56 and 59, the previously traced loop circuit extending between the calling subscriber substation Ti and the line relay R5! in the first selector 5% and to complete; at the contacts 57 and 58, an alternative loop circuit extending between the calling subscriber substation TI and the line relay Pll H in the second selector llfi'. More particularly, ground potenial is applied by way of the upper winding of the line relay RE! I! and the contacts H6 tonne of the line conductors of the trunl; it and by way of the contacts 51 to the line conductor Cfil of the trunk 69, and thence by way of the line switch 35 to the line conductor C35 of the subscriber line 39 extending tothe calling subscriber substation T I and battery potential is applied by Way of the lower winding of the line relay RH! and the contacts M9 to the other line conductor of the trunk its and by way of the contacts to the line conductor 062 of the trunk 6b and thence by Way of the line switch 35 to the line conductor 032 or" the subscriber line 36 extending to the calling subscriber substation Tl. Accordingly, the line conductors C3! and C32 of the subscriber line 39 extending to the calling subscriber substation Tl are poled respectively and by the line relay Rlil in the second selector Hi3; which polarity of the subscriber line (it is without effect at the calling subscriber substation Tl. When this alternative loop circuit is thus completed, the line relay RH! operates to condition the second selector lit! to be responsive to the second digit dialed at the calling subscriber substation Tl and to complete, at the contacts 5 l2, a circuit including the cri-ntacts Ht for extending ground potential into the timer 525; thereby to initiate operation thereof. At this point it is noted that in the event the second digit of the directory number of the called subscriber sub station T3 in the remote exchange 253 is not dialed within a predetermined time interval, the timeroperates in order to shoot operation of the signal relay Rlfit for a purpose more fully explained hereinafter.

The subscribe station Tl tl or" the direct substation T3 by th lineay RUE group of trunk 31 tral oifice id and then to selec ciiicallv, the

trunk extol. Initiating switchi P 25; whci eupcn he the second selector 5 the contacts loop circuit extending scriber substation the second 553. s

' you .-l in to interrupt, t sly trac d ,i vihereby the to be redial 1 of th digits oi the d ectory number of called subscriber subst; l the remote t change 23; whereby the transmitted over is abo o to the trunk circ l5?) and over trunk re to .ltch re us in order to the switcl: ratus 3 to extend the call. to l line extending to called subscriber substation T3 in the remote exchange 20. The subsequent operation of the switching apparatus I45 depends upon the idle or busy condition of the called subscriber substation T3 at this time in accordance with conventional practice. Assuming that the called subscriber substation T3 in the remote exchange 28 is idle at this time, the switching apparatus I45 operates in order to project ringing current over the subscriber line I46 extending thereto; and when the subscriber at the called subscriber substation T3 answers the call by removing the receiver of the telephone instrument thereat from its associated switchhook, the switching apparatus I I-5 switches through in order to complete a connection between the calling subscriber substation T! in the central ofiice I and the called subscriber substation T3 in the remote exchange 28.

The release of this established connection is under the control of the subscriber at the calling subscriber substation TI in the central omce l6, and when the receiver of the telephone instrument thereat is returned to its associated switchhook, the hookswitch I2 is opened; thereby to interrupt the previously traced loop circuit extending between the calling subscriber substation TI and the trunk circuit I50. The trunk circuit I58 is released effecting the release of the second selector Hi], the first selector 53, the line switch 35 in the central office l0 and also the release of the switching apparatus M5 in the remote exchange 29.

In view of the foregoing description of the extension of a trunk call from the private subscriber substation TI service, it will be understood that the extension of a trunk call from aparty subscriber substation rendered non-restricted service will proceed in an identical manner. By way of illustration, it is noted that the extension of a trunk call from the calling party subscriber substation PTI in the central ofiice i9 rendered non-restricted service via the party subscriber line I'll the associated line switch I75, the trunk 58, the first selector 5D, the trunk I 39, the second selector II&, the trunk I60, the trunk circuit iiifl, the trunk line I 40, the switching apparatus i 55 and the subscriber line I45 to the called subscriber substation T3 in the remote exchange 2% is identical to that described above.

Now assume that a call has been initiated at the subscriber substation T2 rendered restricted service in the central oflice I9; that the line switch 45 has operated to select the trunk 68 extending to the first selector 50; that the first selector 50 has operated to select the trunk I39 extending to the second selector I In! and that the second selector Iii) is conditioned to be responsive to the second digit dialed at a calling subscriber substation; and that the subscriber at the calling subscriber substation T2 continues to attempt to extend a connection to a called su scriber substation in the remote exchange 28. At this time the first selector 5D has responded to the first digit of the directory number of the called subscriber substation in the remote exchange in order to select the trunk I38 extending to the second selector I H] as noted above; whereby the line relay RI I I in the second selector IIU has operated to initiate operation of the timer I incorporated therein. Also the line conductors CM and C42 of the subscriber line are respectively poled and by the line relay RI II over the loop circuit extending between the calling subscriber substation T2 and rendered non-restricted the second selector IIO. Accordingly, the device 25 bridging the impulsing springs 24 of the dial 23 at the calling subscriber substation T2 passes substantial current as a consequence of the present poling of the line conductors of the subscriber line 40 from the line relay RII I in the second selector IIG; whereby'the device 25 short-circuits the impulsing springs 24 of the dial 23. Consequently, the impulsing springs 24 of th dial 23 are rendered incapable of transmitting further impulses from the dial 23 at the calling subscriber substation T2 over the subscriber line 40. Thus the subscriber at the calling subscriber substa tion T2 cannot control the line relay RI I I in the second selector I I0; and further operation of the dial 23 at the calling subscriber substation T2 is without effect;

Shortly thereafter, the timer I25 in the second selector IIO completes its timing operation in order to efi'ect operation of the signal relay RI20; whereby the latter relay completes, at the contacts I2 I, an obvious path for connecting a signal tone current supplied by the signal tone transformer I22 to one of the line conductors of the trunk I30; which signal tone current is returned over the loop circuit to the calling subscriber substation T2 indicating to the subscriber thereat that the present call may not be completed to the called subscriber substation in the remote exchange 20. When the subscriber at the calling subscriber substation T2 then replaces the receiver of the telephone instrument thereat upon its associated switchhook, the hookswitch 22 is opened, interrupting the loop circuit extending between the calling subscriber substation T2 and the line relay. RIII in the second selector II 0. The line relay RIII then restores, thereby to effect the release of the second selector IIII, the first selector 50 and the line switch 45 in the manner previously explained. Also upon restoring, the line relay RI I I in the second selector I I0 interrupts, at the contacts I I2, the previously mentioned path for applying ground potential to the conductor extending to the timer I25; whereby the timer I25 is released bringing about the restoration of the signal relay RI20. Upon restoring, the signal relay RI20 interrupts, at the contacts I2I, the previously traced path for returning signal tone current over the trunk I30. At this time, the connection between calling subscriber substation T2 and the second selector III) is released and all of the apparatus employed therein is completely released and available for further use.

In view of the foregoing description of the manner in which a trunk call initiated at the private subscriber substation T2 rendered restricted service in the central oflice I0 is blocked in the second selector IIO, it will be understood that a trunk call initiated at a calling party subscriber substation rendered restricted service in the central oflice I0 such, for example, as the party subscriber substation PTZ will be blocked in the second selector I II! in an identical manner.

Also, it will be appreciated that in the event the subscriber at a calling subscriber substation rendered non-restricted service initiates a trunk call and causes the extension thereof to the second selector II 0 and then fails to dial the second digit of the directory number of the called subscriber substation in the remote exchange 20 that the timer I25 in the second selector III] is operated in the manner explained above in order to effect operation of the signal relay RI 20 therein causing the return of signal tone current to the calling subscriber substation mentioned; all in the manner previously explained.

Calls to the operator position 100 in the central oflice 10 Since private subscriber substations rendered restricted service T2, etc., and party subscriber substations rendered restricted service PT2, etc., are not permitted to extend trunk calls directly from the central ofiice I to called subscriber substations in the remote exchange 20, this result can be accomplished at the calling subscriber substations mentioned only with the aid of the operator at the operator position I00; and subscribers at calling subscriber substations rendered restricted service should extend connections directly to the operator position I60 and obtain the assistance of the operator thereat in the extension of calls to called subscriber substations in the remote exchange 20. However, the subscriber at any calling subscriber substation in the central oifice [0 may extend a call to the operator position I08 in order to obtain the assistance of the operator thereat in setting up toll calls or for information or assistance.

For example, the subscriber at the calling subscriber substation Tl rendered non-restricted service in the central oflice I0 may extend a connection by way of the subscriber line 30, the line switch 35, the trunk 60, the first selector 50 and the incoming trunk 90 to the trunk circuit 95 terminating the trunk 96 extending to the operator position its in an obvious manner. When the connection is thus extended to the trunk circuit 95, a signal is transmitted over the trunk 95 to the operator position Hill; and when the op erator thereat answers the call, a connection is completed to the calling subscriber substation Tl. Thereafter, when the subscriber at the calling subscriber substation Tl replaces the receiver of the telephone instrument H thereat upon its associated switchhook, the hooksvvitch I2 is opened to interrupt a loop circuit substantially identical to that previously traced between the calling subscriber substation TI and the trunk circuit 95; whereby the trunk circuit 95 is released effecting the release of the first selector 59 and the line switch 35. Also, the trunk circuit 95 transmits a release signal over the trunk 98 to the operator position I051: and the operator at the last mentioned position then takes down the connection to the trunk 96.

In view of the foregoing description of the extension or a, connection from the calling subscriber substation TI' rendered non-restricted service in the central office II! to the operator position I00 therein, it will be understood that calls from the subscriber substations T2, PTI, PTZ etc., in the central office H! may be extended in an identical manner to the operator position I00 therein. In passing it is noted that calls from subscriber substations rendered restricted service such, for example, as the subscriber substations T2 and PM in the central oifice Hi to the operator position I90 therein, may be extended since dialing beyond the first selector 5!] is not required in the extension of the calls mentioned,

Conclusions In view of the foregoing, it is apparent that both restricted and non-restricted subscriber substations, whether they are connected to individual or party lines, may selectively control the automatic switching apparatus to complete local calls in the central office it. However, only non-restricted subscriber substations, whether they are connected to individual or party lines, may selectively control the automatic switching apparatus to complete trunk calls to subscriber substations in the remote exchange 28-. Moreover, it will be appreciated. that any subscriber substation, whether it is connected to an individual line or a party line may be readily converted from non-restricted service to restricted service merely by changing the dial thereat, providing the subscriber sub-station with a dialhas an asymmetrical current conducting device bridging the impulsing springs of the dial.

While there has been described what is at present considered to be the preferred embodimentof the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention What is claimed is:

l.v In a telephene system including a first change provided with subscriber substations of first and second classes and a first switch accessi-ble to said subscriber substations and a second switch accessible to said first switch and having access to said subscriber substations and a third switch accessible te said first switch, a second exchange, and a trunk extending between said exchanges and accessible to said third switch; each of said subscriber substations including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit, each. of said subscribe-r sub-stations of said second class also including an asymmetrical current conducting device bridging the impvlsing contacts of the associated. digit transmitting mechanism and arranged to block the passage therethr'ough of substantial current of a predetermined polarity, means including the digit transmitting mechanism at any calling subscriber substation for operating said. first switch selectively to extend connections to said second switch and to said third switch, means included in said second switch for supplying current of said predetermined polarity to the circuit of the connected calling subscriber substation so that said second switch can be operated under the control of the digit transmitting mechanism at a calling subscriber substation of either of said classes in order to extend a connection to a called subscriber substation, and means included in said third switch for supplying current of the opposite polarity to the circuit of the connected callin subscriber substation so that said third switch can be operated under the control of the digit transmitting mechanism at a calling subscriber substation of said first class to extend a connection to said trunk but cannot be operated under the control of the digit transmitting mechanism. at a calling subscriber substation of said second class to extend a connection to said trunk.

2, In a telephone system including subscriber substations of first and second classes, a first switch accessible to said subscriber substations, and second and third switches accessible to said first switch; each of said subscriber substations including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit, each of said subscriber substations of said second class also including an asymmetrical current conducting device bridging the impulsing contacts of the associated digit transmitting mechanism and arranged to block the passage therethrough or" substantial current of a predetermined polarity, means including the digit transmitting mechanism at any calling subscriber substation for op" erating said first switch selectively to extend connections to said second switch and to said third switch, means included. in said second switch for supplying current of said predetermined polarity to the circuit of the connected calling subscriber substation so that said sec ond switch can be operated under the control of the digit transmitting mechanism at a calling subscriber substation of either of said classes, and means included in said third switch for sup plying current of the opposite polarity to the circuit of the connected calling subscriber substation so that said third switch can be operated under the control of the digit transmitting mechanism at a calling subscriber substation of said first class but cannot be operated under the control of the digit transmitting mechanism at a calling subscriber substation of said second class.

3. In a telephone system including a subscriber substation, a first switch accessible to said subscriber substation, and second and third switches accessible to said first switch; said subscriber substation including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit and an asymmetrical current conducting device bridging said impulsing contacts and arranged to block the passage therethrough of substantial current of a predetermined polarity, means including said digit transmitting mechanism for operating said first switch selectively to extend connections from said subscriber substation to said second switch and to said third switch, means included in said second switch for supplying current of said predetermined polarity to said circuit when a connection is extended thereto from said subscriber substation so that said second switch can be operated under the control of said digit transmitting mechanism, and means included in said third switch for supplying current of the opposite polarity to said circuit when a connection is extended thereto from said subscriber substation so that said third switch cannot be operated under the control of said digit transmitting mechanism.

4. In a telephone system including subscriber substation, a first switch accessible to said subscriber substation, and second and third switches accessible to said first switch; said subscriber substation including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit and an asymmetrical current conducting device bridging said impulsing contacts and arranged to block the passage therethrough of substantial current of a predetermined polarity, means including said digit transmitting mechanism for operating said first switch selectively to extend connections from said subscriber substation to said second switch and to said third switch, means including a line relay in said second switch for supplying current of said predetermined polarity to said circuit so that said line relay is operated when a connection is extended from said subscriber substation to said second switch and can be selectively controlled by said digit transmitting mechanism, and means including a line relay in said third switch for supplying current of 15 v the opposite polarity to said circuit so that said last-mentioned line relay is operated when a connection is extended from said subscriber substation to said third switch but cannot be selectively controlled by said digit transmitting mechanism.

5. In a. telephone system including a subscriber substation, a first switch accessible to said subscriber substation, and second and third switches accessible to said first switch; said subscriber substation including a circuit and a digit transmitting mechanism provided with inipulsing contacts serially arranged in said circuit and an asymmetrical current conducting device bridging said impulsing contacts and arranged to block the passage therethrough of substantial current of a predetermined polarity, means including said digit transmitting mechanism for operating said first switch selectively to extend connections from said subscriber substation to said second switch and to said third switch, means included in said second switch for supplying current of said predetermined polarity to said circuit when a connection is extended thereto from said subscriber substation so that said second switch can be operated under the control of said digit transmitting mechanism, means included in said third switch for supplying current of the opposite polarity to said circuit when a connection is extended thereto from said subscriber substation so that said "did switch cannot be operated under the con trol of said digit transmitting mechanism, and timin means controlled by the failure of said third switch to operate within a predetermined time interval after a connection is extended thereto for returning a signal over the connection to said subscriber substation.

6. In a telephone system including a subscriber substation, a switch accessible to said subscriber substation and first and second channels accessible to said switch; said subscriber substation including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit and an asymmetrical current conducting device bridging said impulsing contacts and arranged to block the passage therethrough of substantial current of a predetermined polarity, means including said digit transmitting mechanism for operating said switch selectively to extend connections from said subscriber substation to said first channel and to said second channel, means included in said first channel for supplying current of said predetermined polarity to said circuit when a connection is extended thereto from said subscriber substation so that a subsequent digit from said digit transmitting mechanism can be transmitted into said first channel, and means included in said second chann l for supplying current of the opposite polarity to said circuit when a connection is extended thereto from said subscriber substation so that a subsequent digit from said digit transmitting mechanism cannot be transmitted into said second channel.

'7. In a telephone system including a party line serving subscriber substations of first and second classes, a first switch accessible by way of said part line to said subscriber substations, and 4 second and third switches accessible to said first switch; each of said subscriber substations including a circuit and a digit transmitting mechanism provided with impulsing contacts serially arranged in said circuit, each of said subscriber substations of said second class also including an asymmetrical current conducting device bridging 17 the impulsin-g contacts of the associated digit transmitting mechanism and arrangement to block the passage therethrough of substantial current of a predetermined polarity, means including the digit transmitting mechanism at any calling subscriber substation for operating over said party line said first switch selectively to extend connections to said second switch and to said third switch, means included in said second switch :for supplying current of said predetermined polarity to said party line and thence to the circuit of the connected calling subscriber substation so that said second switch can be operated over said party line under the control of the digit transmitting mechanism at a calling subscriber substation of either of said classes, and means included in said third switch for supplying current of the opposite polarity to said party line and thence to the circuit of the connected calling subscriber substation so that said third switch can be operated over said party line under the control of the digit transmitting mechanism at a calling subscriber substation of 18 said first class but cannot be operated over said \party line under the control of the digit transmitting mechanism at a calling subscriber substation of said second class JOHN WICKS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

